I cannot log in to dashboard or see blog, 500 error….

Blog was fine this morning after adding new plug ins last night and generally upgrading the blog http://www.giblogswallop.co.uk but now when I try to log into the dashboard I get a ‘500 – Internal server error.
There is a problem with the resource you are looking for, and it cannot be displayed.’ and this is not just the dashboard but anyone trying to look at the blog…………….I am lost as to why